High pressure sodium lamps containing unsaturated fills of sodium and mercury are known to the art, as are lamps which use electrodes that include thorium oxide as an emissive material. Such lamps have frequently suffered from a loss of sodium as a constituent of the arc stream which is confined within the arc tube during operation of the lamp. The loss of this sodium reduces the luminosity of the lamp.
Sodium loss can be attributed to several factors. For example, arc tubes that are used for this type of lamp are commonly formed of polycrystalline alumina which can be shaped in a tube-like configuration and sealed at the ends with caps, also formed of alumina. The caps are fused to the envelope with a glass-ceramic frit. These lamps operate at temperatures such that the sodium of the fill can strip oxygen from the enclosed gaseous atmosphere, the alumina of the envelope, the emissive cathode coating, and also from the frits that are used to form the seals. A sodium oxide reaction product is formed by the stripping. This sodium oxide compound will deposit on the arc tube wall in layer-like configuration and will reduce the transmission of light through the envelope. Also, the stripping of the oxygen from the alumina produces a greying discoloration on the inside of the arc tube. This discoloration also reduces the luminosity of the lamp. Thus, it is desirable to remove as much of the oxygen from the arc tube, so as to eliminate these problems.
As generally practiced, before a lamp is fabricated, as much of the oxygen as possible is removed from the unassembled parts, usually by firing them in a hydrogen atmosphere or under vacuum. It has also been conventional to compensate for the sodium which will be lost during operation of the lamp due to the reaction with the oxygen. This approach requires the lamp maker to "saturate" the arc tube with a significant excess of sodium, such lamps having quantities of sodium and mercury sufficient to allow a pool of these metals to remain in the arc tube, even when the lamp is operating.
One known attempt to reduce the sodium loss problem is an effort to reduce the level of oxygen impurity of the lamp which, in turn, reduces the sodium loss since there is a shortage of oxygen for combination with the sodium. Such an effort is set forth in U.S. Pat. No. 4,075,530 by Furukubo et al. wherein a decomposable material, NaN.sub.3, is located in an adjacent exhaust tube, heated to decompose the NaN.sub.3 and cooled to condense to a resultant material while the undesired nitrogen gas is withdrawn. Obviously, such a process is relatively cumbersome of apparatus and expensive of labor and materials.
Other attempts to remove oxygen have included the disposition of a getter within the arc tube of the lamp, such as disclosed in the co-pending U.S. Application, Ser. No. 473,895 filed Mar. 10, 1983 and assigned to the same assignee as the present application. Another approach has been to use electrodes including thereon an emissive material having a free energy of formation per mole of oxygen more negative than the free energy of formation of barium oxide, such as described in the co-pending application, Ser. No. 858,552, filed Apr. 23, 1986, which is a continuation application of Serial No. 539,605, filed Oct. 6, 1983, now abandoned, and assigned to the same assignee as the present invention.
Other approaches for removing the deleterious oxygen have included coating a getter material on a substrate and disposing a disc of the coated substrate on the shank of the electrode, such as disclosed in the co-pending application, Ser. No. 473,897 filed Mar. 10, 1983, and assigned to the same assignee as the present invention.
Japanese Kokai Patent No. SHO 57(1982)-9044 of Mitsubishi Denki K.K., based on application No. 515-83057 of June 19, 1980, discloses the preparation of an electrode for a discharge lamp formed from thoriated tungsten containing thorium dioxide as an electron emitting material.
In the European patent application to General Electric, No. 0,193,714 of Jan. 17, 1986, the use of an emissive coating of calcium, barium, tungsten oxide having a specific composition defined in terms of the triaxial plot of FIG. 3 thereof with zirconium, hafnium and yttrium metal included as a getter is disclosed.